The need for better astronomical and ground based telescope resolution has driven the manufacturing of larger diameters of primary mirrors of such telescopes. However, larger diameter primary mirrors result in the primary mirrors having additional weight and manufacturing problems. For example, because large solid mirror blanks weigh more, they require more time to cast and to anneal. The heavier mirror blanks also bend under their own weight, and are more difficult to maneuver in the factory.
In contrast, lightweight mirrors, fabricated from lightweight mirror blanks, have the advantage of increasing the frequency of the first resonant mode. Light weighting mirror blanks, as it is termed in the industry by those skilled in the art, make the mirror assembly more tolerant of spacecraft maneuvers, as well as increasing the mirror's stability. Lightweight mirrors also result in lighter payloads and lower booster rocket power requirements.
Different inventors have suggested various methods of light weighting mirror blanks. One approach takes a high quality front plate and attaches it to a foam core. For mechanical stiffness, a back plate was usually added to the rear of the foam core. U.S. Pat. No. 4,670,338 issued Jun. 2, 1987 to Alain Clemino and titled “Mirror Foamed Glass Substrate And Method Of Manufacture” discloses a series of foamed blocks glued together and then attached to face sheets. In U.S. Pat. No. 5,208,704 issued May 4, 1993 to Richard R. Zito and titled “Ultralight Mirrors,” a fibrous substrate made from silica and alumina fibers was sealed and subsequently coated with a slurry glaze. The coefficients of thermal expansion (CTE's) were matched to prevent warping. Tatsumasa Nakamura, et al. disclose in U.S. Pat. No. 5,316,564 issued May 31, 1994, and titled “Method For Preparing The Base Body Of A Reflecting Mirror,” a process to fuse a thin plate to foamed silica using a silicon-rubber curing agent. Nakamura, et al. also disclosed fusing the thin plate using fine glass powder. In U.S. Pat. No. 5,640,282 issued Jun. 17, 1997 to Yoshiaki Ise, et al., and titled “Base Body Of Reflecting Mirror And Method For Preparing The Same,” the inventors disclose attaching a high-quality plate to a porous substrate using silica powders. Claude L. Davis, Jr., et al. (U.S. Pat. No. 6,176,588, issued Jan. 23, 2001, and titled “Low Cost Light Weight Mirror Blank”) show an optical surface attached to extruded ceramic honeycomb (e.g., Corning's CELCOR®) with room temperature vulcanizing silicon. These approaches all use adhesives that have slightly different CTE's. Also, the bonding materials are hydroscopic and can change dimensions with humidity.
A second approach is described in U.S. Pat. No. 3,713,728, issued Jan. 30, 1973 to Lewis M. Austin, et al.; whereby molten glass is poured around small refractories. The refractories (e.g., Glasrock Foam No. 25) were supported by pins. Later, the refractories were removed. This process resulted in a dimensionally stable mirror blank, however, the degree of light weighting with this process is limited.
In a third approach, a core structure is built up from thin struts and face sheets are attached to the strut structure. U.S. Pat. No. 4,917,934, issued Apr. 17, 1990 to Daniel R. Sempolinski, and titled “Telescope Mirror Blank And Method Of Production” discloses a strut assembly with frit bonding and then bonds the assembly to face plates with frit bondings or tape cast strips. These frit bonds are subject to moisture absorption. Also, struts tend to sag when heated, unless the struts are thick. Thick struts will limit the degree of possible lightweighting. Phillip R. Martineau, in U.S. Pat. No. 6,045,231, issued Apr. 4, 2000, and titled “Open Core Light-Weight Telescope Mirror And Method Of Manufacture” disclosed front and back plates fused to a strut structure by fusing the plates at the softening point. The strut structure is open to the outside diameter, eliminating the need for vent holes. Concerns remain that this design suffers from stability problems especially when the optic is mounted in a trunion or tip/tilt mount.
The Hextek Company has successfully made mirrors using their GAS-FUSION® process. In this process, borosilicate glass tubes are pressurized while the tubes are heated between face sheets. The tubes are pressed into a hexagonal close-pack geometry. The temperature is reduced and the pressure is reduced. The result is an 85% light-weight core. While this process yields a structurally sound blank, the industry is now demanding still lighter mirrors. The degree of light-weight is limited by the cells supplying enough structural support after heating and before inflating. Cells too thin will sag after heating.
Russian Patent No. 739458 from Derevensky, et. al. shows closed tubes with spherical bulges. The inventors disclose arranging the tubes such that the spherical regions are in a close-packed orientation, however, the tube arrangement is not maximally dense. The parts are fabricated from sealed tubes. Regions along each tube are heated and blown. Each tube needs to be a custom length and while there may be sets of equal lengths, tubes cannot be fabricated until the overall mirror blank dimensions are known.
Located on the Internet at www.kodak.com, Eastman Kodak Company combined the core structure approach with a low temperature fusion (LTF) process to make several mirrors. The core structure is cut from a solid blank using an abrasive water jet (AWJ) tool. The face sheets are fused to the polished core structure and a back plate is added. However, the LTF process may still be improved upon to reduce manufacturing time and process costs.
In these aforementioned mirror blank fabrication processes, a supplier requires custom tooling and significant time to build the mirror blank to specification. The costs for tooling, material, and process steps can be prohibitive.